This invention relates generally to the electrochemical analysis of oxygen, and more particularly to a submersible probe adapted to continuously analyze the concentration of oxygen dissolved in a liquid.
In liquid wastes, the factor which determines whether biological changes are being brought about by aerobic or by anaerobic organisms is dissolved oxygen. Aerobic activity requires free oxygen and produces innocuous end products, whereas anaerobic activity can utilize chemically bonded oxygen such as sulfates to produce end products which are obnoxious. Because both types are ubiquitous in nature, it is vital in waste treatment that conditions conducive to aerobic activity be encouraged, for otherwise anaerobic organisms will take over.
Thus in aerobic treatment processes intended to purify sewage and industrial wastes, the present practice is to continuously measure the dissolved oxygen in order to monitor and maintain proper aerobic conditions. Since all aerobic treatment techniques depend upon the presence of dissolved oxygen, the continuous testing thereof is essential when regulating the rate of aeration, not only to insure that the supply of oxygen is adequate to maintain aerobic conditions, but also to prevent excessive use of energy needed for aeration.
The need for dissolved oxygen measurement is by no means limited to sanitary engineering, for oxygen is a significant factor in iron and steel corrosion, such as in steam boilers. Thus in control systems for removing oxygen from boiler-feed waters, it is customary in the power industry to measure the dissolved oxygen concentration.
Dissolved oxygen probes of the electrochemical type are well known. Some of these probes exploit the magnitude of the depolarizing effect of oxygen on a special galvanic cell. Thus in U.S. Pat. Nos. 3,510,421 and 3,239,444 there are disclosed embodiments of electrochemical cells which are immersible in liquid for measuring the concentration of dissolved oxygen. In its simplest form, the cell is constituted by an anode and a cathode bridged by an electrolyte. The cell is adapted by means of a diffusion membrane permeable only to gases but impermeable to liquids to receive a sample of oxygen. Upon the entry of the sample, a chemical reaction occurs, modifying the electrical characteristics of the cell.
In U.S. Pat. No. 3,239,444, the consumable anode is fabricated of cadmium and the inert cathode is of gold, the electrolyte being an aqueous solution of sodium chloride. The elctrochemical reaction in this cell produces cadmium hydroxide which, as pointed out in this patent, is only soluble to a slight extent and appears on the anode as a deposit which increases the electrical resistance of the cell. As a consequence, the cell after a period of time becomes ineffective and the deposit must be removed in order to again render the cell operative.
U.S. Pat. No. 3,510,421 also recognizes that the electrochemical cell disclosed therein for measuring dissolved-oxygen is subject to a loss of output current over an extended period of time, this being due to the build-up of particles formed from insoluble precipitates within the electrolyte which eventually limit the flow of output current. The reduced cell output causes the cell to lose its calibration and to, in time, become altogether inoperative. In order to overcome this drawback, the patent provides a filter to trap the insoluble particles. Though a filter will act to prolong the effective life of the cell, in time the filter becomes clogged and the operation of the cell is impaired.
Thus known types of submersible electrochemical cells for measuring dissolved oxygen give rise to serious practical difficulties when used continuously as sensors in waste treatment and other systems in which a control function is carried out, for the loss of accuracy which occurs with the build-up of insoluble precipitates is disturbing to the proper operation of the system. While this can be avoided by frequent cleaning of the cells, this requires that the system be periodically shut down for this purpose.